A particulate matter may be coated with another material to protect the surface thereof or confer specific properties thereto by various coating processes, e.g., a gas phase coating process which is advantageous in that a uniform coating layer can be obtained. A liquid phase coating process called a sol-gel process, on the other hand, requires complicated procedures and often gives an irregular coating layer.
Such a gas phase coating process has been adopted in the formation of composite microparticles. For example, U.S. Pat. No. 5,268,337 discloses a method for the formation of composite microparticles from two gaseous precursors having different reaction rates using a counterflow diffusion flame burner. However, this method is not suitable for the mass production of composite microparticles. Also, a gas phase coating method for preparing TiO2—SiO2 composite microparticles using a continuous flow hot-wall aerosol reactor has been reported, which comprises forming TiO2 particles and coating the TiO2 particles with SiO2 by way of separately introducing suitable precursors thereof at different positions of a reactor (See Powell, Q. H. et al., J. Mater Res., Vol 12, No. 2, pp. 552–559 (1997)). However, this method is not suitable for preparing ultra-fine composite particles (nanoparticles) because it requires a long residence time of the precursors in the reactor besides the problem of the precursor deposition on the reactor wall.